The electrolysis of aqueous solutions of ionizable chemical compounds, particularly brines, in an electrolytic cell equipped with an anode and a cathode separated by a diaphragm is well-known in the art. A variety of materials have been tested and used as anodes in such electrolytic cells. In the past, the material most commonly used for this purpose has been graphite. However, the problems associated with the use of graphite anodes are several. The chlorine overvoltage of graphite is relatively high, in comparison for example with the noble metals. Furthermore, in the corrosive media of an electrochemical cell, graphite wears readily, resulting in substantial loss of graphite and the ultimate expense of replacement as well as continued maintenance problems resulting from the need for frequent adjustment of spacing between the anode and cathode as the graphite wears away. The use of noble metals and noble metal oxides as anode materials provides substantial advantages over the use of graphite. The electrical conductivity of the noble metals is substantially higher and the chlorine overvoltage substantially lower than that of graphite. In addition, the dimensional stability of the noble metals and noble metal oxides represents a substantial improvement over graphite. However, the use of noble metals as a major material of construction in anodes results in an economic disadvantage due to the excessively high cost of such materials.
In attempts to avoid the use of the expensive noble metals various other anode materials have been proposed for use as coatings over valve metal substrates. In U.S. Pat. No. 3,627,669, it is disclosed that mixtures of tin dioxide and oxides of antimony can be formed as adherent coatings on a valve metal substrate to form an anode useful in electrochemical processes. In the electrolytic production of chlorine, anodes of this type provide the advantage of economy in the elimination of the use of expensive noble metals or noble metal oxides. In addition the tin oxide coating provides an effective protection for the substrate. However, the tin oxide compositions, although useful as an anode material, exhibit a chlorine overvoltage that is substantially higher than that of the noble metals or noble metal oxides. Thus, despite the elimination of expensive noble metals, the cost of chlorine production, in processes using such anodes, is relatively high.
Considerable effort has been expended in recent years in attempts to develop improved anode materials and structures utilizing the advantages of noble metals or noble metal oxides. A great amount of effort has been directed to the development of anodes having a high operative surface area of noble metal or noble metal oxide in comparison with the total quantity of the material employed. This may be done, for example, by employing the noble metal as a thin film or coating over an electrically conductive substrate. However, when it is attempted to minimize the aforementioned economic disadvantage of the noble metals by applying them in the form of very thin films over a metal substrate, it has been found that such very thin films are often porous. The result is an exposure of the substrate to the anode environment, through the pores in the outer layer. In addition, in normal use in an electrolytic cell, a small amount of wear, spalling or flaking off of portions of the noble metal or noble metal oxide is likely to occur, resulting in further exposure of the substrate. Many materials, otherwise suitable for use as a substrate are susceptible to chemical attack and rapid deterioration upon exposure to the anode environment. In an attempt to assure minimum deterioration of the substrate under such circumstances, anode manufacturers commonly utilize a valve metal such as titanium as the substrate material. Upon exposure to the anodic environment titanium as well as other valve metals, will form a surface layer of oxide which serves to protect the substrate from further chemical attack. The oxide thus formed, however, is not catalytically active and as a result the operative surface area of the anode is decreased.
In the electrolytic cells of the prior art, it is known to provide a porous diaphragm separating the anode and the cathode and thereby minimizing the flow of liquids from the anode compartment to the cathode compartment of the cell. However, in most instances, such cells are operated under conditions such that ionic migration and molecular migration through the porous diaphragm occurs to a substantial degree resulting in the contamination of the cathode liquor with undecomposed electrolyte and of the anode liquor with reaction products of the cathodic materials and anodic materials. More recently it has been found that many of the disadvantages of the porous diaphragms of the prior art may be overcome through the use of membrane diaphragms which are impervious to both liquids and gases and which provide a control of both ionic and molecular migration during electrolysis. Such membrane diaphragms fabricated from synthetic organic ion-exchange resins are disclosed, for example in U.S. Pat. Nos. 2,967,807, 3,390,055, 3,852,135, and French Pat. No. 1,510,265. Among the resins disclosed for use as membrane diaphragms are included, for example, cation exchange resins of the "Amberlite" type, sulfonated copolymers of styrene and divinyl benzene and others.
It is also known from co-pending application Ser. No. 513,376 filed Oct. 9, 1974, that improved diaphragms for use in electrolytic cells may be prepared from a copolymer of tetrafluorethylene and a sulfonated perfluorovinyl ether. Diaphragms of this type represent a substantial advantage over the previously known membrane diaphragms with respect to retention of effectiveness, that is inertness to the electrolyte and products of the electrolysis, over extended periods of operation.
It is a primary object of this invention to provide a novel and improved electrolytic apparatus and method whereby electrolysis of aqueous solutions of ionizable chemical compounds may be carried out over extended periods of operation with improved efficiency and maintenance characteristics.
It is another object to provide a novel electrolytic apparatus utilizing as the anode thereof, an improved anode having an operative surface of noble metal or noble metal oxide and having improved efficiency and maintenance characteristics.
It is a further object of this invention to provide a novel electrolytic apparatus utilizing as the diaphragm a material which precludes or substantially reduces both molecular migration and undesirable ionic migration, but which still permits the conduction of electrical current by movements of desirable ions.
A further object is to provide novel electrolytic apparatus and process employing an improved permselective diaphragm and an improved anode, which can be operated efficiently over long periods without destruction of the diaphragm or rapid deterioration of the anode.
Other objects and advantages will be apparent to those skilled in the art on consideration of this specification and the appended claims.